The compression cylinders of such piston compressors, which are designed for pressures of around 20 bar to 450 bar, often are no longer fabricated as sand mold castings.
Forged steel blocks or, for low pressure ranges as far as borehole diameters of around 500 mm, also cast iron blocks, chill molds, or simple model casting blocks are used as semifinished blanks.
In this kind of cylinder design, all gas spaces and channels that are formed by the inserting of sand cores during customary model casting are introduced by sometimes costly boreholes or milling work during the mechanical fabrication.
Thus, e.g., for dual-action compression cylinders, the connection of the valve chambers at the top cover and crank side has to be produced by a longitudinal borehole.
These connection boreholes have to be closed gas-tight at the end face of the compression cylinder by covers—similar to the valve covers. This gas-tight design requires a costly design solution, especially at high operating pressures.
The suction and pressure ports are arranged in the region between the particular valve chambers or directly on the respective valve chambers. Access to the space around the compression cylinder is greatly impaired by the pressure and suction pipes leading away from the suction and pressure ports.
FIG. 1 shows a dual-action compression cylinder 10 of the prior art.
The compression cylinder 10 has a cylinder chamber 25 in the middle, in which the piston 30 is arranged to move in the longitudinal direction. On either side of the piston 30 are formed two compression spaces 20a, 20b. At the end face, this cylinder chamber 25 is closed gas-tight by a cylinder cover 13. The compression cylinder 10 has a suction port 18 in the cylinder wall 24 and opposite it a pressure port 19, each of them emerging into a connection channel 17a and 17b, respectively. At both ends of the connection channels 17a, 17b, extending in the lengthwise direction parallel to the lengthwise axis, are arranged valve chambers 15a to 15d with valves 16a to 16d, being connected to the cylinder chamber 25 by corresponding channels. For reasons of fabrication, the connection channels 17a and 17b are open at the end face to chambers 22a and 22b, adjacent to the valve chambers 15b, 15c. Thus, they must be closed by separate covers 14a, 14b at the front end face 12a. At the rear end face 12b, the cylinder space 25 is closed off by means of a cover 26 with a gasket for the piston rod 32. In another design, the cylinder space 25 is configured as a blind borehole. In this case, the piston rod is led through the cylinder bottom by means of a smaller borehole. Thus, the closing cover 26 is omitted.
The side ports 18 and 19 have a considerable influence on the outer dimensions of the compression cylinder 10. Major drawbacks of this design are large size of the semifinished blanks and greatly impaired access when the compressor layout is arranged vertically. For horizontal compressors, whose compression cylinders are often braced against the foundation, the tanks lying underneath the pressure pipe often hinder this bracing.